New discoveries improving a physician's ability to treat neoplastic diseases, i.e. cancer, are continually being made. For example, superior tumor growth rate inhibiting drugs are being discovered and new therapeutic regimens for concurrent administration of drugs are being designed. Difficulties sometimes are encountered when developing tumor growth rate inhibiting drugs because the drugs have to be capable of performing their function at acceptable levels of toxicity in vivo.
Creatine is a compound which is naturally occurring and is found in mammalian skeletal muscle, brain, and other organs (except for the liver). Creatine can be synthesized relatively easily and is believed to be non-toxic to mammals. Kaddurah-Daouk et al. (WO 92/08456 published May 29, 1992 and WO 90/09192, published Aug. 23, 1990) describe methods of inhibiting the growth, transformation and/or metastasis of mammalian cells using drugs "capable of inhibiting purine metabolic enzyme activity". Examples of drugs described by Kaddurah-Daouk et al. include cyclocreatine, homocyclocreatine, 1-carboxymethyl-2-iminohexahydropyrimidine, guanidino acetate and carbocreatine. Kaddurah-Daouk et al. also studied ten other guanidino-like analogs and concluded that the ten analogs lacked the ability to inhibit the growth, transformation or metastasis of mammalian cells. This conclusion was based on the results of an in vitro assay using known tumor cell lines.